In recent years, a polymerase chain reaction (PCR) intended for detecting specific fungi, bacteria, viruses, or other organisms has enhanced its popularity in the medical, veterinary, food, and other fields because the analysis results can be known in a short period of time such as about 2 hours. However, a technology for detecting unspecified fungi, bacteria, viruses or other organisms in a short period of time has yet been established.
Very great merits are expectable when a trace of an unspecific organism can be detected, identified, and quantified from a place which should be in a sterile environment in nature. For example, blood, cerebrospinal fluid, amniotic fluid, urea, or the like can be used as a sample for analysis to early detect and identify the infection of humans or domestic animals, leading to the administration of an effective antibiotic at an early stage. In addition, the state of recovery can be monitored using the quantitative value of infecting bacteria. Great merits in the field of quality control of daily life water, foods, cosmetics, and the like can also be expected. For example, the presence of undesired, unspecified bacteria, fungi, viruses or other organisms can be quickly detected and identified in daily life water which persons have a possibility of inhaling (drinking) in life. Such water includes, tap water, water from a water tank, air-conditioning circulating water, water from a humidifier, hot spring water, and swimming pool water, foods, and cosmetics. In addition, the presence level can be monitored with high sensitivity. Thus, it is assumed that establishment of a method for simply and rapidly quantifying or identifying a subject microorganism to be detected in a sample with high sensitivity renders the rippling field of the technology very wide; thus, there is strong need therefore.
Sepsis is a serious systemic infection and in whose definite diagnosis the detection/identification of a causative microorganism in the blood is mandatory. The number of patients having sepsis has recently increased with the sophistication of medical treatment such as cancer treatment or organ transplantation. In view of in-hospital infections, multidrug-resistant bacteria including methicillin-resistant Staphylococcus aureus (MRSA) often constitutes a causative bacterium of sepsis; thus, to select a suitable antibiotic for the life saving of patients, it is clinically important to detect and identify a causative microorganism in the blood as rapidly as possible.
Intrauterine infection, which is the most common cause of premature birth, is a serious infection fatal to fetuses; thus, it will be important for the life saving of fetuses to detect and identify a causative microorganism thereof in the amniotic fluid as rapidly as possible, and to administer the most suitable antibiotic at an early stage of the occurrence thereof. Similarly, in the veterinary field, bovine mammitis is a very serious disease for milk cows, for example; when the treatment thereof is delayed, there is often no means other than removal, also leading to industrial problems.
However, culture methods using culture bottles and selective media are typically used in current detection methods for infecting microorganisms. They take at least several days to obtain the results thereof. Thus, clinically, at present, empiric therapy is forced to be carried out until the results are revealed. As a result, an antibiotic is forced to be blindly selected, which represents a major disadvantage, while the detection is required to be rapid. Some microorganisms may have antibiotic resistance genes. Therefore, a drug susceptibility test is often performed in parallel; however, it takes several days to produce results as with the detection method for identification. As a result, the appearance of multidrug-resistant bacteria due to the use of broad-spectrum antibiotics and the inappropriate choice of antibiotics cause a situation, for example, that patients with sepsis and fetuses with intrauterine infection cannot be saved; and that milk cows with mastitis are forced to be removed. In addition, the detection of heterotrophic bacteria has a high risk of producing false-negative results because it needs special culture conditions.
Against such a background, the detection of unidentified bacteria has been studied using PCR: an attempt has been made to detect and identify a causative microorganism of sepsis by amplifying a trace of DNA of the causative microorganism by PCR; and hybridizing the amplified causative microorganism DNA to a strain-specific nucleotide probe targeted at an empirically assumed microorganism (JP06-90799A). In addition, the development of an detection technique for sepsis using real-time PCR employing hybridization probes as a basic principle has been studied for more rapid detection/identification of a causative microorganism (Journal of Analytical Bio-Science, Vol. 28, No. 5 (2005) 400-404). A rapid detection/identification method for a causative microorganism has been studied by performing gene amplification by PCR using microorganism DNA as a template and a specific primer set, and then analyzing the combination of melting temperatures (Tm values) of the resultant products, specific for microorganisms or the difference between the Tm values (WO2007/097323). However, accuracy must also be ensured in the results obtained in a short period of time using PCR. Thus, for PCR, it can also be said to be important to make sensitivity compatible with specificity. These prior techniques apply gene amplification techniques using PCR, but they are methods limited to assumed target microorganisms. Thus, they cannot detect microorganisms when outside the scope of the assumption. Even when they are used as detection/identification methods for unidentified microorganisms, a technique for quantifying them has not been established and has been impossible.
Real-time PCR is a sole method through which a curve of amplification with time can be displayed. Therefore, today, it provides a crucial detection technique for the quantitative determination of gene expression. Particularly, detection methods using intercalators such as SYBR Green are world-widely and frequently used, because they have low cost and are simple and convenient. However, the real-time PCR using an intercalator detects not only a target but also non-specific amplification products, posing a problem that the detection sensitivity thereof is decreased. A particularly problematical non-specific amplification product thus formed is a primer dimer. To suppress the formation of a primer dimer, various means are proposed by devising design of primers, using the Hot Start method, an amplification method using modified primers (JP2002-291490A), a Hot Start PCR using an improved reagent for PCR (JP2003-259882A), and a method involving adding a substance binding to the primer dimer to a sample (JP2006-254784A). However, it is extremely difficult to completely inhibit the formation of non-specific amplification products including a primer dimer. Even when various methods for suppressing the formation of the primer dimer are used, non-specific amplification products are detected depending on the increased number of PCR cycles, which is the major factor for the decreased sensitivity in the quantitative measurement using the real-time PCR. Even in qualitative detection, the Tm (melting temperature) value must be checked in each measurement to exclude “false-positive” due to the primer dimer, for example, which has become a major problem for the real-time PCR measurement system.
To provide a DNA polymerase used for PCR, a method for producing a DNA polymerase preparation using a genetic recombination technology has been studied (JP2006-180886A). Among commercial thermostable DNA polymerase preparations commonly used for PCR reaction, high purity preparations are also commercially available; however, even in the PCR reaction using each of these high purity preparations, non-specific amplification products of unknown origin are detected, for example, when the gene amplification reaction needs to be performed in conventional (about 30) cycles or more, which has limited the use thereof.
Various techniques have been developed to secure high specificity in the PCR method. The simplest method is a nested PCR method, which, however, needs the labor and time of performing PCR two times. Accordingly, a “nested amplification method” which involves carrying out nested PCR by a single PCR process (JP05-292968A) is proposed. This method is an excellent method in which nested PCR can be performed using only a single thermal cycling profile; however, it has not yet been put to practical use possibly due to the absence of technique by which the Tm values of primers and an amplification product could be easily measured at the time of the application. This method can be put to practical use at present that a real-time PCR technique is available. Other methods such as a method using Hybri-Probe and a TaqMan PCR method are generally used; however, the preparation of probes used for these methods is not easy for everyone and the cost for the preparation thereof is also expensive. Therefore, at present, there not yet exists such a method satisfying all of rapidity, simplicity, and economical efficiency, has been not provided yet.
As described above, when simply amplifying DNA from a trace of a sample microorganism and analyzing, in particular, performing the quantification or identification analysis of the DNA in a short time can be carried out, the analysis of even a trace level of a gene previously incapable of being analyzed can be achieved. In addition, rapid and accurate determination in the fields of medicine, veterinary, and analysis of various samples such as daily life water and foods can be carried out. Meanwhile, however, in PCR for the amplification of a trace of sample microorganism DNA, the control of both sensitivity and specificity to high degrees has not yet been achieved and rapid quantification or quantification/identification for unidentified microorganisms has also not yet been achieved.